1. Field of the Invention
This invention relates to a printing ink composition, and, more particularly, to a printing ink composition suitable for use either in a jet printing apparatus wherein the ink jet assemblies are connected to a pressurized ink source, or in a jet printing apparatus wherein the ink jet assemblies are connected to an ink source under small positive, small negative or zero static pressure, such as in, for example, an ink jet printer of the type disclosed in U.S. Pat. No. 3,683,212.
In early developments in the ink jet printing art the basic imaging technique involved the use of one or more ink jet assemblies connected to a pressurized source of ink. In later developments, however, for example in U.S. Pat. No. 3,683,212 to Zoltan, the ink source is under small negative or zero static pressure. The ink from the ink source is drawn into the ink jet by the capillary action of the ink. Each ink jet has a very small orifice, typically on the order of 0.003 inch. Uniform droplets of ink may be electrostatically charged as they exit from the jet, to be controllably directed onto the recording medium to form alphanumeric characters in response to an electrostatic deflection system.
In the early work relating to ink jet printing, drafting pen inks and fountain pen inks were used. However, these inks were unsuitable for use in jet printing because the solid particles in the dyes of these inks would clog the small orifice of the ink jet. Also, these inks had a tendency to "tip-dry" when left in the ink jet while the ink jet was not in use, e.g., during shutdown periods overnight or for more extended periods when the equipment was not being used. The ink would dry out and form a crust or skin in the orifice of the jet. Upon resumed operation the crust or skin would either prevent ink flow altogether or it would deflect the individual ink droplets emitted from the orifice in an uncontrolled and hence undesirable manner.
It has also been found that the evolution of gases from the ink during the operation of an impulse ink jet printer having an ink source under small positive, small negative or zero static pressure is undesirable. Such gassing inhibits the performance of the printer by interferring uncontrollably with the hydraulic pressure cycle involved in ejecting the stream of ink droplets emitted from the ink jet orifice, and poor printing quality results.
In jet printers generally, and, in particular, in standing impulse jet printers a permeable hose is often utilized as the conduit through which the ink is fed from the ink supply source to the ink jet. The permeable hose is an acoustic dampener which permits better frequency response of the ink so that a single droplet of ink is emitted from the jet tip for each electrical pulse applied to the jet. It has been found that the evaporation of water to the atmosphere of ink compositions comprising a dye in a solvent consisting of a humectant or heavy glycol thinned with water to lower the viscosity to the desired 1 to 10 cps, causes the viscosity of the composition to increase, which, in turn, inhibits or prevents, the desired operation of the printer.
Accordingly, it is an object of the present invention to provide a new and improved printing ink suitable for use in ink printers which substantially reduces or obviates the evolution of gases from the ink composition during operation of the ink jet printer.
It is a further object of the present invention to provide a new and improved printing ink suitable for use in ink jet printing which does not dry out and encrust or skin the jet orifice during the shut down periods of a jet printer.
Yet another object of the invention provides an ink composition which has a stable viscosity when exposed to ambient humidity.
It is yet another object of the present invention to provide a new and improved printing ink suitable for use in ink jet printing which is stable over a long shelf life.
It is still another object of the invention to provide a method for making the new and improved ink composition.
Other objects and advantages of the present invention will be apparent based on the following detailed description of the invention.